The invention relates to a method for changing glass compositions in continuously operated melting installations, in particular for aggressive, high-purity and/or high-melting glasses, and to a melting installation which is suitably modified for this method.
Hitherto, it has been attempted as far as possible to melt each (special) glass which is to be produced in a separate melting installation. Given the very high number of different types of glasses and the widely differing quantities to be melted, the result is a large number of melting installations, each of which requires a corresponding “infrastructure”. Melting installations of this type are often not fully loaded.
It would inherently be less expensive to operate just a single melting installation for various types of glasses. This melting installation would have a significantly improved loading.
Conventional glass changes (generally melt changeover operations) in a melting installation have problems, since both the composition and the physical properties of glasses often have considerable differences. A particular problem with the melt changeover between two glasses consists, inter alia, in the difference in density between the starting glass A and the target glass B. The differences in density delay the melt changeover operation. It is known from the literature that considerable differences in density between starting glass A and target glass B lead to segregations in the melting tank. In this context, the melt changeover from a relatively heavy glass to a lighter glass is particularly critical, since there is a risk that the new glass will simply float on top of the old glass. The result is extremely long melt changeover operations, since the old glass A is not discharged sufficiently quickly from the crucible.
A further problem with melt changeover arises if the starting glass A and the target glass B have very different chemical compositions or belong to different “glass families”. In this case too, segregation and/or devitrification (crystallization) phenomena may occur, and in the most serious circumstances these may not just delay the melt changeover but also make it impossible.
One possible way of avoiding or reducing the melt changeover problems which have just been outlined could involve simply draining the tank and filling it again and melting target glass. However, draining hot molten glass out of the tank involves risks which should be avoided for health and safety reasons. Secondly, particularly in the case of high-purity glasses, the purity demands are so high that even slight remaining glass residues from the starting glass A cannot be tolerated in the target glass B.
Aggressive glasses with a need for a high level of purity are conventionally melted continuously in platinum tanks which include a melting-down tank, a refining tank and a homogenization unit. This melting technology is disadvantageous on account of the high costs of the precious metal, the short service lives of platinum units of this type and the lack of flexibility. In addition to platinum melting-down units, the separate melting installations mentioned in the introduction also use what is known as the skull crucible, which is composed of water-cooled metal pipes arranged at a distance from one another and in which the molten material is heated by means of induction coils, which surround the crucible, by high-frequency energy being radiated in. Melting-down units of this type have the advantage that in the edge region of the crucible, on account of the water cooling, a glass crust is inevitably formed as a protective layer consisting of material of the same type, so that the molten material is surrounded in the form of a crucible “of the same type” and is thereby protected from impurities.
A glass change or the melt changeover between glasses using a skull crucible of this type is not mentioned in the literature. If the melt changeover processes which are conventionally carried out are to be executed in a skull crucible, it must be borne in mind that the glass crust of the crucible, at least in parts, has to be replaced by the target glass, which greatly lengthens the melt changeover operation.
The composition of the target glass is in practice continuously monitored and adjusted in order to maintain the desired specification of the glass. Particularly in the case of highly specific glasses, this adjustment process may take a very long time, for example, a melt changeover time of four days, which entails considerable costs on account of the intermediate glass which is lost.